1. Field of the Invention
The present invention relates to a paper conveying mechanism used in copying machines, printers, etc.
2. Prior Art
As shown in FIG. 9, conventional conveyor mechanisms used in copying machines, etc. basically consist of a conveyor belt 62, which is provided with holes H and driven by a driving roller 61, and a suction fan 63 which is installed beneath the conveyor belt 62. By operating the suction fan 63, an air current C is created which flows from the upper surface A to the conveyor belt 62 toward the lower surface B of the belt 62 via the holes H formed in the belt 62.
When the paper P is fed onto the conveyor belt 62, the holes H on the conveyor belt 62 are blocked by the presence of the paper P and the air current C is thus also blocked. Accordingly, a slight pressure difference is created between the upper surface A and lower surface B. As a result, the paper P is caused to adhere to the conveyor belt 62 by the pressure difference. Then, when the conveyor belt 62 moves, the paper P moves along with the belt.
However, the conveyor mechanism constructed as described above has the following problems. Since the mechanism requires a conveyor belt, suction fan and exhaust duct 64, a large space is required for the installation of these parts. As a result, an apparatus which uses these parts is increased in size. Furthermore, since the parts involve additional expense, the apparatus tends to be more costly.
Furthermore, since the suction-adhesion force which determines the paper conveying capacity of the apparatus depends on the capacity (or strength) of the suction fan, it is necessary to use a large suction fan in order to increase the paper conveying capacity of the apparatus, and this creates problems in terms of further increases in the size and cost of the apparatus.
Moreover, in cases where such a conveyor mechanism is used in electronic photographic devices such as copying machines and printers which use toner, the toner is blown about inside of the apparatus by the turbulence created by the suction fan. This causes problems of toner contamination in the surrounding areas. In addition, the operating noises generated by the suction fan is deleterious on the quiet atmosphere preferred for the office environment.
In the prior art there is also known a transfer belt system which solves the above-described problems. As shown in FIG. 10, this system is equipped with a transfer belt 72 which is used in both the transfer process and the conveying process, a photo-sensitive part 71, a main charger 73, a developing roller 74, and a belt charger 75.
The transfer belt 72 consists of two layers: a dielectric layer on the outer surface side and a conductive layer on the underside. An electric charge is imparted to the outer surface of the transfer belt 72 by the belt charger 75. Paper P is fed onto the transfer belt 72 by conveying rollers 76 and is conveyed to the transfer area D by the transfer belt 72.
When the photosensitive part 71 is irradiated with image exposing light after being, for example, positively charged by the main charger 73, a latent image consisting of a positive electric charge is formed on the surface of the photosensitive part 71. When toner which has been negatively charged is supplied via the developing roller 74, the toner T electrostatically adheres to the latent image area, so that a positive image formed by the toner is developed.
Next, in the transfer area D, an electric field E is formed (see FIG. 11) between the photosensitive part 71 and the transfer belt 72 by the positive charge p on the surface of the transfer belt 72. Accordingly, since the paper P is a dielectric material, when the paper P is conveyed into the electric field E, a negative charge n' is created on the side of the paper P that faces the transfer belt 72 and a positive charge p' is created on the side of the paper P that faces the photosensitive part. Accordingly, the toner which forms a positive image on the surface of the photosensitive part 71 and is negatively charged, is attracted by the positive charge p' on the side of the paper P that faces the photosensitive part 71. As a result, a positive image is transferred to the surface of the paper P.
The paper P onto which the positive image has been transferred is then separated from the surface of the photosensitive part 71, and while still adhered to the transfer belt 72 conveyed into the fixing area (not shown) through the conveying area F.
In the conveying area F (see FIG. 10), the paper P is conveyed while it is still firmly fixed to the transfer belt 72 by the residual negative charge n' on the surface of the paper P and the positive charge p on the transfer belt 72. However, the above described transfer belt system also has inherent problems, i.e. the transfer belt 72 tends to be soiled by the toner.
The reason for this is that the transfer belt 72 which comes into direct contact with the photosensitive part 71 is extremely susceptible to being soiled by toner adhering to the surface thereof except for the areas which have been covered by the paper P. Furthermore, since the toner is electrostatically charged, it shows an especially strong tendency toward adhesion. Moreover, such soiling of the transfer belt 72 leads not only to the paper P being soiled, but also to a loss of the required electrostatic adhesion force. This causes paper jams due to the inability of the paper to adhere to the transfer belt 72.
FIG. 12 shows an improved version of a transfer belt system in which the transfer process and the conveying process are separated (hereunder called "separate process system"). The separate process system differs from the transfer belt system in the following respects: (a) a prescribed gap d is formed between the conveyor belt 92 and the photosensitive part 91 so that the toner on the photosensitive part 91 is not attracted by the charge on the conveyor belt 92 and (b) the system is designed so that transfer (or copying) is accomplished by means of a conventional transfer charger, transfer roller or pressure transfer process.
In such a separate-process system, the conveyor belt 92 is not soiled by toner; thus, even without any special cleaning means being provided for the surface of the belt, the paper is not soiled.
Experiments were performed to ascertain the main factors involved in the electrostatic adhesion force between the conveyor belt 92 and the paper P. It was found that the electrostatic adhesion force is proportional to the charge on the surface of the conveyor 92. However, it was also found that the experimental data representing this adhesion force showed considerable fluctuation according to factors such as the temperature and humidity in which the experiments were performed, the time elapsed after opening the package of the copying paper and the positions of the photosensitive part 91 and the conveyor belt 92. Accordingly, in this system paper cannot be stably conveyed using a minimal charge. As a result, there have been problems in the practical application of this system.
When the causes of the above mentioned variations in the adhesion force in the separate-process system were investigated, the following findings were obtained:
Specifically, since the gap between the photosensitive part 91 and the conveyor belt 92 is large, the electric field is small in inverse proportion to the gap. Accordingly, the quantity of the charge created on the surface of the paper P is extremely sensitive to variations in the dielectric constant of the paper P. As a result, the quantity of the charge created on the surface of the paper P varies with temperature, humidity and the time elapsed following the opening of the copying paper package. Consequently, the force with which the paper adheres to the conveyor belt 92 shows considerable variation.
With the above described conventional apparatuses in mind, there has been a demand for a conveyor mechanism which would not soil the paper with toner and in which stable conveying can be accomplished using a minimal electric charge.